There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products). Accordingly, there is an ongoing need to provide lighting which is more energy-efficient.
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
The most common type of general illumination is white light (or near white light), i.e., light that is close to the blackbody locus, e.g., within about 10 MacAdam ellipses of the blackbody locus on a 1931 CIE Chromaticity Diagram. Light with such proximity to the blackbody locus is referred to as “white” light in terms of its illumination, even though some light that is within 10 MacAdam ellipses of the blackbody locus is tinted to some degree, e.g., light from incandescent bulbs is called “white” even though it sometimes has a golden or reddish tint; also, if the light having a correlated color temperature of 1500 K or less is excluded, the very red light along the blackbody locus is excluded. The 1931 CIE Chromaticity Diagram (an international standard for primary colors established in 1931), and the 1976 CIE Chromaticity Diagram (similar to the 1931 Diagram but modified such that similar distances on the Diagram represent similar perceived differences in color) provide useful references for defining colors as weighted sums of different hues.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light. “White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps use red, green and blue light emitting diodes, and other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. While there is a need for more efficient white lighting, there is in general a need for more efficient lighting in all hues.
Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra).
Representative examples of the use of solid state light emitters to provide a lighting device that provides excellent energy efficiency and excellent duration of service at a reasonable cost are described in U.S. Pat. No. 7,213,940, issued on May 8, 2007. U.S. Pat. No. 7,213,940 discloses lighting devices that comprise:
a first group of solid state light emitters which, if illuminated, would emit light having a dominant wavelength in the range of from 430 nm to 480 nm;
a first group of lumiphors (that comprise luminescent material) which, if excited, would emit light having a dominant emission wavelength in the range of from about 555 nm to about 585 nm; and
a second group of solid state light emitters which, if illuminated, would emit light having a dominant emission wavelength in the range of from 600 nm to 630 nm;
wherein if current is supplied:                a combination of (1) light exiting the lighting device which was emitted by the first group of solid state light emitters, (2) light exiting the lighting device which was emitted by the first group of lumiphors, and (3) light exiting the lighting device which was emitted by the second group of solid state light emitters would, in an absence of any additional light, produce a mixture of light having x, y coordinates on a 1931 CIE Chromaticity Diagram which define a point which is within ten MacAdam ellipses of at least one point on the blackbody locus on a 1931 CIE Chromaticity Diagram, and        a combination of (1) light exiting the lighting device which was emitted by the first group of solid state light emitters, and (2) light exiting the lighting device which was emitted by the first group of lumiphors would, in an absence of any additional light, produce a sub-mixture of light having x, y color coordinates which define a point which is within an area on a 1931 CIE Chromaticity Diagram enclosed by first, second, third, fourth and fifth line segments, the first line segment connecting a first point to a second point, the second line segment connecting the second point to a third point, the third line segment connecting the third point to a fourth point, the fourth line segment connecting the fourth point to a fifth point, and the fifth line segment connecting the fifth point to the first point, the first point having x, y coordinates of 0.32, 0.40, the second point having x, y coordinates of 0.36, 0.48, the third point having x, y coordinates of 0.43, 0.45, the fourth point having x, y coordinates of 0.42, 0.42, and the fifth point having x, y coordinates of 0.36, 0.38.        